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<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
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<body>

  <script>
    let arr = [1, 2, [3, 4], [5, 6, [7, 8, 9]]];
    /**第一种方式：flat */
    let res1 = arr.flat(Infinity);
    console.log(res1);
    let res2=arr.join().split(',').map(Number)
    console.log('res2',res2);
    console.log(arr.toString());
    console.log(arr.join());
    arr.push(...[99,99])
    console.log(...[88,88]);

    let arr2=[6,7,8,8,4,3,6]
    console.log(arr2.filter(item=>
      {item>5}
    ));
    console.log(arr2.map(item=>item>5));
  </script>
</body>

</html>